1. Field of the Art
The present invention relates to a pump system and a method for operation thereof and, more particularly, to a pump system which is installed, for example, in a flow control tank of small-sized combined septic tank equipment.
2. Prior Art
Standardization of small-sized combined septic tanks has heretofore been promoted for the purpose of preventing water pollution of rivers, lakes and marshes. FIG. 8 schematically shows small-sized combined septic tank equipment in which a conventional pump is installed. As illustrated in the figure, sewage flowing into a flow control tank 2 from a raw sewage tank 1 is supplied to an anaerobic tank 4 by a pump 3, and the sewage is purified in the anaerobic tank 4 and an aerobic contact aeration tank 5 and is then discharged.
As the pump 3 employed for the above-described purpose, a rubber vane submersible pump or a small-output, general-purpose submersible sanitary sewage pump or non-clogging sewage pump has heretofore been used.
The above-described rubber vane submersible pump is a positive-displacement pump, which rotates at relatively low speed. Therefore, it has the advantageous feature that a low flow rate can readily be obtained and an approximately constant pump discharge can be obtained independently of the pump head. Accordingly, it is possible to construct small-sized combined septic tank equipment without providing a flow control device 6 for controlling the flow rate of sewage supplied from the flow control tank 2 to the anaerobic tank 4 by the pump 3.
However, in order to reduce the wear of the rubber vane of the rubber vane submersible pump and to thereby ensure a predetermined lifetime, it is necessary to use a special motor having a multipolar structure, such as a 12-pole motor. Thus, the conventional system suffers from the problem that the product cost of the pump is disadvantageously high. In addition, it is impossible to avoid an increase in power consumption due to friction occurring in the rubber vane part, and it is also difficult to overcome internal friction. Accordingly, the conventional system including a rubber vane submersible pump gives rise to the problem that electric power consumption is high and a long operation lifetime cannot be expected. Further, since this type of pump generates a high-pitched noise, the small-sized combined septic tank equipment, which is likely to be installed near a residential area, may cause a noise problem.
On the other hand, when a submersible sanitary sewage pump is used, the following problems arise: The pump discharge required for the pump 3, which is used in the flow control tank 2 of the small-sized combined septic tank equipment, is relatively small for example, 20 lit/min, whereas the pump discharge of a submersible sanitary sewage pump is excessively high; even the smallest of those which are commercially available at the present time has a pump discharge in the order of 100 lit/min. The reason for this is as follows: Since the structure of submersible sanitary sewage pumps is the same as that of general centrifugal pumps, it is difficult to reduce the size of a submersible sanitary sewage pump to achieve a low flow rate while ensuring a choke-proof pumping performance. Accordingly, when this type of pump is used, it is necessary to provide a flow control device 6 on the discharge side of the pump 3, as shown in FIG. 8, to return the greater part of sewage to the flow control tank 2, thereby supplying a controlled amount of sewage to the anaerobic tank 4. Thus, since it is difficult to reduce the size of the above type of submersible pump and the flow control device 6 is needed, electric power consumption is disadvantageously high and the cost of the equipment is relatively high.
Therefore, it has heretofore been demanded to provide a small-sized waste pump system which is conformable to small-sized combined septic tank equipment.
In view of the above-described circumstances, it is an object of the present invention to provide a small-sized pump system which is capable of exhibiting the required pumping performance and of controlling the flow rate and the pump head and which is free from the above mentioned power consumption, cost, wear and noise problems.
Another object of the present invention is to provide a method for operating a small-sized pump system which is capable of accomplishing the same object.